Not applicable.
The present invention relates generally to computer systems and particularly to networking computer systems. More particularly, the present invention relates to implementing a computer network using conventional telephone wiring.
Computer networks permit multiple computer systems to communicate and share data over one or more transmission lines. Although a network may connect computers located anywhere in the world, a smaller local area network (LAN) is commonly used to connect computers located in fairly close proximity, such as within the same room or building. A LAN typically comprises one or more physical wires or cables that connect the computers, although wireless networks are available. In a multidrop LAN, all of the networked computers connect to the same physical cable (or wireless link), either directly or through an intermediate device such as a terminator or adapter. Local area networks have enjoyed widespread popularity in the workplace, where they permit users to share documents and send and receive electronic messages. Local area networks also permit each personal computer to connect with one or more xe2x80x9cfileserverxe2x80x9d computers, which generally provide a central storage location for documents and program files and which often form gateways to other networks, such as the Internet.
Until recently, computer networks in the home have not been widely implemented due to the high cost of purchasing the computers and because of the expense, trouble, and difficulty faced by the average consumer of installing network cables and running the equipment. As computers have become cheaper and more powerful, however, many consumers have purchased multiple computers for the home; and recent developments in computer networking have made it easier to form multidrop home networks by connecting computers through existing home telephone lines. The networked computers then send data to each other over the phone wires. Similar networks rely on standard telephone wiring in businesses, schools, apartments, dormitories, hotels, and other structures which have the wiring in place.
Even though these network devices communicate over telephone lines, they are not required to wait for a dial tone before establishing communication or to follow other standard protocols required for outgoing calls. As a result, the home networking devices can communicate over a phone line that is in use by another service, such as xe2x80x9cplain old phone servicexe2x80x9d (POTS) or Digital Subscriber Line (DSL) service. In order to coexist with other services, the phone line network signals are transmitted at high frequencies which a telephone, POTS modem, or DSL modem does not use. The high frequencies used for phone line networking typically range from approximately 2 million hertz (MHz) to 10 MHz, in contrast with POTS, which occupies the frequency spectrum from 0 hertz to about 4 thousand hertz (kHz). The POTS frequency band is within the audio frequency band, which includes frequencies that the human ear can detect. Digital Subscriber Line (DSL) modems generally communicate in the range of around 100 kHz to approximately 1.5 MHz.
Computers on a phone line network system generally send digital data by (1) transmitting a high frequency xe2x80x9ccarrierxe2x80x9d signal and (2) altering, or modulating, the carrier signal based on the data. The number of possible alterations typically is fixed (hence the xe2x80x9cdigitalxe2x80x9d nature of the data), and each type of alteration represents a particular data value. The computer that is receiving the data monitors the carrier signal and determines the data that was transmitted by detecting, or xe2x80x9cdemodulating,xe2x80x9d the alterations. Current phone line networking equipment uses pulse position modulation, in which the carrier is a short pulse that is transmitted periodically at fixed time intervals. To transmit a particular data value, the transmitting computer alters the timing between two successive pulses by delaying the second pulse relative to the first pulse. Accordingly, the pulses represent the carrier signal, and the delays represent the modulation. By determining the time delay between pulses, the receiving computer can detect which data value was transmitted.
Unfortunately, pulse position modulation tends to be particularly susceptible to electronic interference (called xe2x80x9cnoisexe2x80x9d) picked up by the phone lines. The delay values can be adaptively lengthened to combat the noise, but doing so generally lowers the data throughput, or the speed at which data is transmitted. Although the voltage threshold level can be raised above the noise level, doing so requires the transmitting computer to transmit at higher voltage levels, requiring more expensive signal filtering circuitry and increasing power consumption.
In addition, the analog circuitry currently used to measure the time delays in pulse position modulation signals is not highly sensitive to small time increments. As a result, with conventional circuitry the different delay values available to a single pulse must be spaced somewhat far apart in time, limiting the achievable data rate to no higher than about one million bits (or xe2x80x9cmegabitsxe2x80x9d) per second, a relatively slow throughput compared to that of conventional LAN""s. The analog circuitry also is susceptible to normal manufacturing tolerances in the crystals which control timing in the transmitter and receiver. The crystal in the transmitter controls the intervals between carrier pulses, while the crystal in the receiver controls the circuitry that measures the intervals to decode the data. If the transmitter crystal frequency deviates significantly from the receiver crystal frequency, however, then the receiver may not decode the data properly.
Another problem with pulse position modulation is that, unlike other types of digital modulation techniques, standardized software code is not widely available for implementing pulse position modulation algorithms. As a result, xe2x80x9ctime-to-marketxe2x80x9d can be slow for products that incorporate pulse position modulation. For these and other reasons, pulse position modulation is rarely used as a standard modulation technique in industry and thus suffers from lack of compatibility with many existing systems, dramatically increasing the time and effort required to design and implement a network.
An added difficulty with pulse position modulation is the xe2x80x9cburstyxe2x80x9d nature of the modulated carrier, which means that the maximum voltage of the signal is significantly higher than the average signal voltage. The signal burstiness is related to the fact that the signal reaches its highest level whenever a pulse is transmitted but remains at a zero level during the delays between pulses. Bursty signals place high performance demands on filtering circuitry, requiring greater expense and complexity in the receiver. Unless costly precautions are taken, bursty signals can interfere with normal telephone conversations.
Existing phone line networking systems also suffer from relying on analog comparator circuitry to detect when the transmitted signals exceed the voltage threshold. Although analog circuitry can be cheaper and simpler to implement that digital circuitry in some cases, analog comparators are sensitive to noise, a condition that ultimately can limit the data throughput. The analog circuitry also tends to be relatively difficult and expensive to upgrade as technology progresses. For example, existing analog hardware typically cannot be modified to support standard modulation techniques or higher data rates. The analog circuitry also prevents full duplex communication, which allows computers to transmit and receive at the same time over the network. Instead, current systems operate in half duplex mode, which means that a single computer cannot transmit and receive data at the same time.
Further, the analog nature of existing phone line network equipment makes compensating for unpredictable conditions in phone line quality difficult. Certain telephone cables, for example, may affect certain signal frequencies (or xe2x80x9cfrequency bandsxe2x80x9d) differently. Within certain guidelines, communications devices generally are allowed to select an optimum frequency band before transmitting data. The analog nature of existing phone line network equipment, however, makes it difficult to adjust or control the frequency band. Instead, existing analog equipment must be designed to operate within a fixed frequency spectrum, resulting in degraded performance if the cable operates poorly within the chosen frequency band.
For the foregoing reasons, a telephone line networking system capable of resolving these problems would greatly improve the speed, cost, and complexity of implementing computer networks. Such a system, if devised, should incorporate spectral flexibility, robust noise performance, high data rates, standard modulation techniques, and full duplex communication without requiring an expensive or complex design. The system should also provide backward compatibility with existing pulse position modulation systems and employ accurate synchronization techniques during demodulation. Despite the apparent advantages that such a system would provide, to date, no such device provides these features.
The aforementioned problems are solved by a computer system with a networking modem capable of full duplex communication over standard telephone lines. The networking modem comprises a digital signal processor (DSP) capable of implementing a plurality of digital modulation and demodulation techniques, including pulse position modulation and quadrature phase shift keying (QPSK). The pulse position modulation ensures backward compatibility with existing phone networking systems, while the QPSK modulation provides a standard modulation technique that permits efficient development and cross platform compatibility. The networking modem may be configured to implement other modulation techniques as well, including quadrature amplitude modulation (QAM). The DSP may comprise a commercially available DSP or an application specific integrated circuit (ASIC), and may be either programmable or nonprogrammable, as desired.
The use of a digital signal processor permits a wide variety of processing options, including the capability to adjust the spectral content of the transmitted signals, resulting in maximum channel capacity and data rates. Outgoing symbols can be appropriately shaped or selected to compensate for channel conditions on the telephone line, thus reducing intersymbol interference (ISI). In addition, digital filtering and correlation processing permit robust noise performance through noise averaging. To achieve accurate synchronization during demodulation, the DSP implements a digital phase locked loop (DPLL) that recovers the timing of the incoming carrier signal. Digital techniques also permit the DSP to implement echo cancellation, further reducing ISI and permitting full duplex communication. Due to the reduced ISI and improved noise immunity, the DSP can transmit successive data symbols more closely spaced in time, thus achieving higher data rates than were possible using prior art analog receivers.
The DPLL comprises a numerically controlled oscillator (NCO) that generates a local timing signal, along with a phase detector and loop filter that compare the incoming carrier signal to the local timing signal. The loop filter provides an error signal to the NCO, and the NCO adjusts the frequency of the local timing signal until phase lock occurs with the incoming carrier signal. In a preferred embodiment, the DSP implements a carrier detector that asserts a carrier detect signal to indicate phase lock of the incoming carrier with the local timing signal. The carrier detector monitors the error signal provided by the loop filter, asserting the carrier detect signal if the error signal amplitude falls within a predetermined range. Under pulse position modulation, the DSP implements an interval counter that increments during each cycle of the local timing signal, if the carrier detect signal is deasserted. Accordingly, the counter measures the length of time between pulses of the carrier, thus providing an effective and accurate symbol detector. Because the DPLL synchronizes the counter to the incoming carrier, problems with clock drift due to differences in transmitter and receiver crystal tolerances are eliminated. The DPLL also permits a more robust and frequency selective carrier detector than do prior art techniques, which typically utilize less reliable energy detection techniques.
In addition to the digital signal processor, the networking modem comprises line coupling magnetics and amplifiers for transmitting and receiving analog signals over the telephone line, a CODEC that includes an analog to digital converter (ADC) and a digital to analog converter (DAC), and a media access controller (MAC) for interfacing to a computer system or directly to a local area network. The CODEC samples incoming analog signals received through the line coupling magnetics and passes the resulting incoming digital signals to the DSP. The DSP demodulates the incoming digital signals to detect the transmitted signals. By synchronizing directly to the timing of the incoming carrier, the DPLL eliminates the need to synchronize the receiver""s local clock source with the transmitter""s clock source, thus providing extremely accurate symbol timing without relying on complex and expensive synchronization techniques. The DSP provides detected symbols to the computer system via the MAC, which preferably interfaces to a PCI bus within the computer.
The DSP also receives outgoing data symbols from the computer system via the MAC and modulates the data symbols onto a carrier signal, using any of a variety of modulation techniques. Because the DSP handles multiple modulation and demodulation schemes, the networking modem is capable of communicating with a variety devices over the same telephone line, even if each device uses a different modulation scheme. In a preferred embodiment, the DSP maintains a lookup table with a variety of waveforms corresponding to each transmitted symbol. From the lookup table, the DSP selects optimum waveforms based on the spectral conditions of the telephone line, thus maximizing the channel capacity for an optimum data rate. In addition, the DSP is capable of dynamically switching symbol waveforms in order to compensate for evolving channel conditions. The DSP is capable of transmitting a first waveform to represent a symbol, for example, and subsequently transmitting a second waveform to represent the same symbol, depending on the channel conditions at the time of each transmission. The CODEC converts outgoing digital signals received from the DSP into outgoing analog signals, which are passed to the telephone line via the amplifiers and line coupling magnetics.
In an alternative embodiment, the DSP is capable of handling modulation and demodulation not only for phone line networking, but also for standard modem communications and/or DSL. The embodiment preferably comprises a V.90 data pump, a DSL data pump, and a networking data pump incorporated into one or more DSP""s. The data pumps generally manage all processing for standard modem communications, DSL communications, and phone line networking, including modulation and demodulation/detection. The data pumps communicate through an input/output port to exchange transmitted and received symbols with an external computer. A filter bank, including a POTS filter, a UASDL filter, and a network filter, performs bandpass filtering on signals traveling between the phone line and the data pumps. A decimator receives sampled signals from the phone line and creates a custom sample stream at the appropriate rate for each data pump. The DSP further includes an interpolator that combines outgoing modulated signals from the data pump, at a sample rate suitable for transmission over the phone line.
Thus, the present invention comprises a combination of features and advantages that enable it to substantially advance the art by providing a networking modem that incorporates spectral flexibility, robust noise performance, high data rates, standard modulation techniques, full duplex communication, and backward compatibility with existing pulse position modulation systems, while eliminating problems with symbol synchronization during demodulation. These and various other characteristics and advantages of the present invention will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention and by referring to the accompanying drawings.